On the role of the ND-rich phases in sintered ND-FE-B magnets

Sintered Nd15-Fe77-B8 magnets contain a multiphase microstructure. Our analytical transmission electron microscope study reveals that the Nd-rich phase, which is an essential part of the liquid phase sintering process can be divided into at least 4 subtypes with different Nd:Fe-ratios. Impurities of the raw material such as Nd-oxides and Nd-chlorides are randomly found in magnets, partly in the form of small inclusions up to 500 nm diameter within 2:14:1-hard-magnetic grains and partly in the form of large grains up to several microns diameter. As consequence of this analytical study a high concentration of boron vacancies as well as iron vacancies, especially in grain boundary regions is proposed. We have found that the degree of continuity of the intergranular Nd-rich phase is proportional to the intrinsic coercivity of the magnet. The intrinsic coercivity of sintered Nd-Fe-B magnets is determined by the nucleation of reversed domains and by the expansion of the reversed muclei, which is hindered by the non-magnetic Nd-rich oxide grain boundary layer phase.